The present invention relates to silicone pressure sensitive adhesives compositions, hereinafter also referred to as SPSAs, which cure by the action of ambient moisture, and to a method for their preparation. In a preferred embodiment the present invention relates to SPSA compositions which cure to a permanent adhesive which are useful as a structural adhesive.
By the term "silicone pressure sensitive adhesive" it is meant an adhesive comprising one or more siloxane components and possessing sufficient tack and cohesive strength so that it can be adhered with mild pressure to a clean stainless steel substrate and then removed therefrom and adhered to the same or another clean substrate. By the term "permanent adhesive" it is meant an adhesive that bonds to a clean substrate and cannot be so removed and thereafter re-adhered.
At the present time all commercial SPSA compositions are designed for the relatively low strength bonding of surfaces, such as electrical insulating tapes, medical tapes and devices and high temperature masking tapes. These commercial SPSA compositions are also designed to possess their ultimate adhesive properties when first applied.
However, there is a need for a SPSA which will provide higher bonding strengths and/or which will develop additional adhesive properties, such as resistance to flow or long term durability, after being applied.
Silicone structural glazing of curtainwall panels represents one of those needs. Durable structural glazing is currently done with silicone sealants in a multi-step, labor-intensive, time-consuming process. Significant improvements in the structural glazing process could be realized if a SPSA were used in place of the silicone sealant in the current process.
Unfortunately, standard SPSAs are not completely suitable for structural glazing applications because of their inherent flowability, albeit slow in the recently improved SPSAs, under sustained stress. Under the sustained stress of gravity the heavy glass panel that is currently used in structural glazing, if supported only by pressure sensitive adhesive, would eventually experience shear failure of the bond. A SPSA that cures to a non-shearing adhesive having high adhesive strength would be very beneficial in this and other applications.
In copending U.S. patent application Ser. No. 08/063,105, now U.S. Pat. No. 5,470,923 to Krahnke et al., commonly owned, the applicant's set forth a moisture-curable silicone pressure sensitive adhesive comprising (1) a mixture of a silicone resin and polydiorganosiloxane gum wherein the resin and/or gum bears alkoxy or alkoxyalkyl radicals or (2) a bodied product of the resin/gum wherein the bodied product bears alkoxy or alkoxyalkyl radicals. The compositions of Krahnke et al. are produced by reacting a silicone pressure sensitive adhesive composition comprising a resin/gum or bodied product wherein the resin, gum or bodied product has reactive alkenyl, hydroxyl, or hydrogen atoms with a moisture-reactive compound which is capable of reacting with the alkenyl, hydroxyl or hydrogen atoms in the SPSA. The compositions of Krahnke et al. initially exhibit properties of a pressure sensitive adhesive however, upon exposure to moisture they cure to a permanent adhesive.
It is an object of this invention to show another composition for obtaining moisture-curable silicone pressure sensitive adhesives which become a permanent adhesive in the presence of ambient moisture and which are suitable for structural glazing applications.